Match the flash tool to the firmware's era
A 2015 Xperia came to the bench stuck in a bootloop. Someone had tried a modern build over an old 5.1.1 stock base. The phone never came back up.
On the cable, it wasn't dead. The flash tool could still reach it. But where the device name should have been, the log printed Unknown and a loader date. That line is where most people give up and order a new phone.
It doesn't mean the phone is dead. It means the tool couldn't handshake the phone's loader. The phone is usually right there, in flash mode, waiting. You fix the handshake, not the phone.
The ladder that brought it back, in order:
Flash-mode discipline. The phone only listens if it enters the mode properly: cable in while holding volume down, green light, nothing else running. Sounds too dumb to matter. It's the first thing to rule out.
The USB side. Old flash-mode drivers go stale or hide as ghost entries. Delete the unknown ones in Device Manager, rescan, reinstall the flash-mode driver. A bad driver looks exactly like a dead phone from the chair.
The era rule. Flash tools and firmware bases age together. An old loader won't handshake with a current tool generation. Here: the old base needed Flashtool 0.9.18.6. The Nougat-era firmware needed 0.9.36.0. Match the tool generation to the firmware generation and the handshake comes back.
Once the old full-stock base was back, the phone stopped being a brick. Nougat went on. Then a current-era build went on clean. Both boot. Which one it ends up living on is a taste question, not a rescue question.
The order I'd tell anyone holding a bootloop: full stock first, from the right era, then climb. Don't stack a modern build on a broken old base. And don't take the tool's word about the phone's health. It can't read what it can't handshake.
If you're stuck on a device right now, tell me what it's doing. First read is free.
Andrew, from the bench. paper.wf/andrew-61